Shells thickness, required

Having determined the bending moments, the stress in a given shell, or the shell thickness required to resist the bending moment may be calculated by substituting the value of or in place of in equations (11-32) and (ll-32a). 

For preliminary design purposes only, for the outer shell of a steel-lead-steel packaging, the following equation may be used to estimate the shell thickness required to resist failure when the package is subjected to the penetration test ... 

Tank Shell. Another example of where thickness is set by minimums for fabricabihty but not for strength is in small-diameter tanks. For example, a water storage tank built using a steel of an allowable stress of 20,000 psi (138 mPa), 9 ft (3 m) in diameter by 21-ft (7-m) high, requires a shell thickness to resist hoop stress of only 0.023-in. (0.58-mm) thick. However, if built to API Standard 650, the shell would be fabricated at least 0.1875-in. (4.76-mm) thick. The code requires this thickness so that when fabrication, welding, and tolerances are considered, a tank of acceptable quaUty and appearance meeting the requirements of most services in most locations is provided. 

Other methods for estimating the cost of vessels and fractionators can be used, but weight is usually the best. The cost of fractionators can be correlated as a function of the volume of the vessel times the shell thickness, with an addition for the cost of trays based on their diameter (Reference 13). Fractionator costs can also be correlated based on the volume of the vessel with the operating pressure as a parameter. This requires a great deal of data and does not give as good a correlation as weight. Hall et al. (Reference 14) present curves of column diameter vs. cost. 

The analysis of the membrane stresses induced in shells of revolution by internal pressure gives a basis for determining the minimum wall thickness required for vessel shells. The actual thickness required will also depend on the stresses arising from the other loads to which the vessel is subjected. 

For a cylindrical shell the minimum thickness required to resist internal pressure can be determined from equation 13.7 the cylindrical stress will be the greater of the two principal stresses. 

Density and Specific Gravity. Water has a density, mass per unit volume, of about 62.4 lb/ft3 (1.000 g/cc) at 0°C, whereas mercury, also a liquid, has a density of about 842 lb/ff3 (13.5 g/cc) at the same temperature. All things being equal, greater densities mean thicker required tank shell thicknesses. 

The thickness required for the column-shell increases directly with the pressure and with the column diameter. For a given vapour-handling capacity, increasing the total... 

Tb is the thickness of the shell at the bottom, Tp is thickness required for the operating pressure, D is the diameter of the shell and tray, L is tangent-to-tangent length of the shell... 

The required shell thicknesses are calculated according to the formula suggested in American Standard ASME VIII (see Appendix... 

Minimum thickness (t,) of the shell cylinder required for pressure loading ... 

The wall thickness required for the shell is obtained from the equation given in Ref. E4 ... 

The theoretical model may be used to optimize the cure cycle for a given formulation. Wall temperature and the required shell thickness are the most important parameters that determine the process time. 

Hot shell designs are used in particularly high temperature applications where high, perhaps greater than 0.5 wt %, sulfides and/or sulfates are present in the process stream. They are impractical, however, for low temperature processes such as 1200°F or less due to limitations in materials and thickness required to achieve the hot shell. In these cases, it is not uncommon to insulate the shell externally to raise its temperature above the dewpoint. 

Equation 4-36 is for membrane shells with a negligible thickness. As the pressure and the shell thickness increase, the stress distribution across the thickness is non-uniform. Therefore, some correction to the membrane theory is required. The modified code equation as given by the ASME Unfired Pressure Code, Section VIII, Division 1 is ... 

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